Earth formation logging system



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Dec. 15, 1959 J. J. ARPS 4 2,917,704

EARTH FORMATION LOGGING SYSTEM Filed May 24, 1954 '4 Sheets-Sheet 1 Fir G. 451

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Dec. 15, 1959 J. J. ARPS EARTH FORMATION LOGGING SYSTEM 4 Sheets-Sheet 2 Filed May 24. 1954 lll/llll/llll/ll IN V EN TOR.

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Ill/11111111!!! United States Patent EARTH FORMATION LOGGING SYSTEM Jan J. Arps, Dallas, Tex.

Application May 24, 1954, Serial No. 431,734

25 Claims. (Cl. 3241) This invention relates to logging systems employed in conjunction with the drilling of earth boreholes to provide information concerning the nature of the earths formations adjacent the borehole. More specifically, the invention relates to a logging system which provides such information as borehole drilling proceeds, and which operates to provide accurate information concerning not only the formation traversed by the borehole, but also information concerning one or more unpenetrated formations below the bottom of the borehole and toward which the drilling is proceeding.

Previous successful earth borehole logging systems have required suspension of drilling operations and the removal of the drill string and bit while logging apparatus including a system of electrodes was traversed down and back through the length of the borehole by means of an auxiliary conductor cable. Such logging systems suffer several disadvantages in addition to the expense and inconvenience attendant upon removal of the drill string. It is, for example, prohibitively expensive and time consuming to withdraw the drill string on every occasion when the operator would like to have information concerning the most recently drilled formation. Accordingly, drilling below or beyond a most desirable depth is of common occurrence, many times necessitating expensive auxiliary and additional operations. Additionally, in the interval between cessation of drilling and lowering of the electrode system to the bottom of the borehole, there occurs an invasion of the newly drilled formation by the borehole fluid, and this considerably reduces the distinctness and accuracy of the information furnished by the logging operation.

Some efforts have heretofore been directed toward development of a practical system of logging a borehole during progress of the drilling operation, in an attempt to obviate the aforementioned disadvantages and objectionable features of conventional borehole logging systems. Systems thus far proposed for logging while drilling have, however, been found to be impractical due to complexity, requiring electric conductors in the drill string, lack of speed or sensitivity of the response of the system to changes of earth formation, or other reasons. Further, none of the proposed systems have been capable of providing one desirable type of information; namely, information concerning the nature and relative closeness of a formation interface, or a different earth formation lying below the formation encircling the bottom of the borehole. To illustrate, it is always of value, and sometimes of very great value, to the driller to know what type of formation the bit is approaching as drilling proceeds. Thus, if drilling is proceeding through an oilbearing sand formation but the bit is approaching a salt water bearing formation, it is of great value to know not only the type of formation being approached, but the nearness of the bit to such formation, since it would normally be desirable to stop drilling prior to penetration into the water-bearing formation.

The present invention provides a borehole logging system which obviates the previously mentioned disadvantages and defects of the prior known logging systems and which efficiently produces by simple means, and during drilling operations, a continuous flow of undelayed information concerning the characteristics of the earth formation laterally adjacent the drill bit, prior to appreciable invasion thereof by the drilling fluid; and information concerning the formation or formation interface below the bottom of the borehole and not yet penetrated by the bit. This system also includes a novel mode of determining information concerning the formation or formations below the bottom or end face of the borehole. The system does not require an electrical circuit between the apparatus in the borehole and the apparatus located outside the borehole at the earths surface, the information obtained at the bottom of the borehole being transmitted by other means, preferably by pressurechange signals produced by means in the lower end of the drill string in the downwardly flowing stream of drilling fluid and transmitted by and through the fluid to a signal receiving apparatus outside the borehole.

Generally, the system of the invention utilizes the drill bit, insulated from the drill string thereabove for a considerable distance, as a primary electrode in apparatus for obtaining information concerning the earth formations; and a ground connection remote from the electrode, which connection may conveniently be provided by the uninsulated and therefore grounded portion of the drill string immersed in the borehole fluid and exposed abo e the insulated lower end of the drill string. The aforementioned drill bit electrode, which in some aspects of the invention may be supplemented by one or more auxiliary or secondary electrodes located above and closely adjacent the drill bit electrode, is used in conjunction with the remote ground connection and other apparatus to secure one or more desired types of information concerning the earth formation or formations adjacent the bottom of the borehole. While, as hereinafter more fully explained, various types of information may be secured and utilized within the scope of the invention, information concerning the natural potential of the earth and of the electrical resistance or resistivity of the earth formations is used in illustrating the principles of the invention as exemplified by a selected preferred embodiment of physical apparatus according to the invention and depicted in the accompanying drawings and described and explained in the following specification. Preferably, but not necessarily, the information secured is of two types and is preferably, but not necessarily, procured during the entire course of the actual drilling operation. The information secured is translated to a location outside the borehole and there translated into suitable senseperceptive indications for use by the driller or operator by means and procedures to be hereinafter more fully explained.

The ability of the novel system of the invention to pro vide the driller or operator with information in advance concerning the approach of the bit or the end face of the borehole toward a change in the earth formation through which drilling is proceeding is based upon the fact that different types of formations have different electrical characteristics, such as different electrical resistivities, different natural potentials between spaced-apart points, etc. Thus, the ability to conduct electric current when subjected to a potential gradient varies among different types of formations. As an example, a salt water bearing sandstone proves to be a good electrical conductor, whereas an oil bearing sand would prove to be a poor conductor, and the natural potentials in these different types of formations would be widely different. The system of the invention utilizes one, and preferably more, of the differences in electrical characteristics displayed by the various formations through which and toward which the borehole is being drilled. As illustrative examples, the resistance of the formation to passage of an alternating current, and the natural potential provided by the formation may be cited; and these examples of characteristics will be used in illustrating and explaining a preferred embodiment of the system of the invention. It will, however, be understood that other electrical characteristics exhibited by the formations could be employed, either in place of, or in conjunction with those cited, as will readily be perceived by those skilled in the art as the description proceeds.

To provide an indication of an electrical characteristic of a formation as yet undrilled, and situated beyond the end face of the borehole, an electric current may be constrained or induced to flow along a path which in the region closely adjacent the end of the borehole, is principally beyond the end face of the borehole, and which path in other regions may be situated at considerable distances from the borehole and through great volumes of earth, especially if a large and relatively remote return electrode is employed for the current to flow to through the earth. Since the principal and only appreciably variable portion of the resistance of such a path is that presented by the material closely adjacent the bottom face of the borehole, it is readily seen that variations in the electrical resistance of such electric current path accurately indicate variations in the resistance of the formation material directly beyond and closely adjacent the bottom face of the borehole. Thus, if during drilling of the borehole the resistance of such restricted electric current path were to increase over a period of time (that is, during drilling of a length of the borehole), it would be indicative of approach of the end face of the borehole toward a formation of higher electrical resistance characteristic than that previously being drilled; and corn versely. The information thus obtainable would obviously be of great value to the driller, comprising, as it were, an advance notice of the type of formation about to be drilled into; and this is especially true in those regions where the resistance characteristics of various types of earth formation are known from previous exploration and drilling.

While information of the type hereinbefore described and concerning the as yet undrilled formation is furnished the driller or operator by a system according to the invention, additional information is preferably also supplied which greatly enhances the usefulness and value of that previously described. For example, if the resistance characteristic of the formation beyond the end face of the borehole and being approached by the drill be chosen as the desired characteristic, information concerning the same characteristic of the formation generally or substantially surrounding the bottom of the borehole is capable of greatly enhancing the accuracy of predictions and conclusions regarding the as yet undrilled formation below the bottom face of the borehole; and the same may be said of information concerning the natural potential characteristic exhibited by the formation laterally adjacent the bottom of the borehole. A manner and a means by which information concerning these additional and supplementary characteristics may be obtained will be, by way of example, more fully described hereinafter.

It being an important object of the invention to provide one or more types of information for the driller or operator during drilling operations and without materially interfering with the drilling process or equipment, that part of the apparatus especially provided for the system of the invention is incorporated into and integrated to some extent with conventional drilling equipment. For example, the drill bit, by employment of suitable insulation means, is employed as an electrode through which current may be induced to flow downwardly (that is, ahead) through the undrilled formation beyond the bottom or end face of the borehole, and similarly, by employing insulation means suitably disposed, an upper portion of the drill string exposed to the drilling fluid in the borehole may be utilized as a remote ground connection for return flow of the current passed through the bit and the earth formation. Instrumentalities for providing current, for measuring potentials or current, or resistance or other electric quantity or characteristic, or any combination thereof, may be housed in a chamber provided in a drill collar or other suitable part of the drill string, together with necessary auxiliary apparatus. In this manner, normal functioning of the drilling apparatus is not affected. Indications or signals representing the desired information obtained by the instrumentalities in the drill collar or string at the bottom of the borehole may be produced and suitably translated to a location outside the borehole to there be presented as suitable sense-perceptive indications for evaluation by the driller. Means for performing such auxiliary functions are provided in the disclosed embodiment of this system of the invention. Preferably, other means are also provided; notably means for recurrently restricting an electric current to a path or paths of flow which, in the region closely adjacent the end face or bottom of the borehole, are directed principally downwardly below the bottom face of the borehole (that is, in the direction of extension of the borehole). These other means will hereinafter be fully explained.

It is, accordingly, an important object of the present invention to provide an earth borehole logging system capable of providing information during drilling of the borehole and without interfering with the operation of the drilling of the borehole, whereby the driller or operator may determine, in advance, the approach of the end face of the borehole toward an interface between earth formations of differing characteristics.

It is another object of the invention to provide a method of determining nearness of approach of the end face of an earth borehole to an interface between earth formations of differing electrical characteristics during drilling of the borehole.

[t is another object of the present invention to provide in an earth borehole logging system adapted for logging during drilling, means for logging an electrical characteristic of the formation generally surrounding the drill bit and for separately logging an electrical characteristic of the formation beyond or ahead of the end face of the borehole, whereby the nature and nearness of an interface between formations of differing electrical characteristics being approached by the end face of the borehole may be determined.

It is another object of the present invention to provide a method of determining the approach of the end face of an earth borehole toward an interface between earth formations of different electrical characteristics during drilling of the borehole, whereby the driller may suspend drilling operations, if desired, prior to drilling through such interface.

It is another object of the present invention to provide in an earth borehole logging system adapted for logging during drilling, means for logging the formation ahead of the end face of the borehole whereby approach of the bit toward changes in the earth formation through which an extension of the borehole is directed may be determined in advance.

The above objects, and other objects and features of novelty which will hereinafter be made apparent, are accomplished by the invention, a preferred form of apparatus and mode operation of which are explained and illustrated in the following description considered in con junction with the accompanying drawings, in which like reference numerals indicate like parts in the several views, and in which:

Figure 1 is a partly diagrammatic view illustrating a typical environment and physical arrangement of apparatus according to the invention and including a graphic record of the type produced by the apparatus;

Figures 2A and 2B are views, principally in section, along line 22 of Figure 1 and with a section broken away, illustrating one physical arrangement of apparatus in the vicinity of the lower end of a drill string, including a preferred arrangement of electrodes according to the invention;

Figures 3A and 3B are diagrammatic views illustrating two types of electric current distribution effected in a typical practice of the invention as drilling proceeds through a thick earth formation;

Figures 3C and 3D are views similar to Figures 3A and 3B, but illustrating a changed electric current distribution as the drill bit approaches an interface between an earth formation of low resistivity and a formation of high resistivity;

Figures 4A and 4B are, respectively, a view in section of a portion of an earth borehole being drilled by a rotary drill rig as the drill bit aproaches a formation of high resistivity while still in a formation of lower resistivity; and a graph constructed from information secured by apparatus according to a preferred embodiment of the invention as drilling proceeds through the formations depicted in Figure 4A; and

Figure 5 is a schematic circuit diagram including a schematic representation of apparatus according to the illustrated embodiment of the invention and illustrative of a preferred selected arrangement of electrical and auxiliary components employed in the apparatus in the borehole, with duplicated parts omitted for clarity.

Referring now to the drawings, and to Figure 1 in particular, there is illustrated a typical earth borehole being drilled through successive earth strata or formations, including formations 11, 12, 13, 14 and 15, by means including a drill bit means 16 comprising a drill bit 17 secured to the lower end of a drill collar 18 which forms the lower section of an otherwise conventional drill string 19 comprising one or more sections of drill pipe, and a kelly 20. The kelly and drill string are suspended from a rotary swivel 21 carried by a travelling block 22 supported for vertical movement by a cable 23 rigged in conventional manner about a crown block 24 supported by a suitable derrick 25, the cable extending to and operated by a draw works 26 from which draw works power is provided through bevel gearing 27 for rotating the kelly, drill string, and drill bit means. The drill bit means, drill string, kelly and swivel are provided with one or more suitable internal passages through which is pumped, under pressure, a stream of drilling fluid supplied to the swivel through a rotary hose 28 and a conduit 29 by a pump 30, the hose permitting travel of the swivel, kelly, drill string and drill bit means as the borehole is extended during the drilling operations.

Pump 30 is provided with a suitable surge bell or tank 31"se"Tving to reduce pulsations in the discharge of the pump, and draws the drilling fluid from any suitable source such as a supply sump 32, through an intake pipe 33. The drilling fluid forced downwardly through the drill string and out orifices in the drill bit returns to the surface through the annular space encircling the drill string, carrying with it the drill chips, and is discharged from the upper cased portion of the borehole through a suitable means such as pipe 34, through which the fluid is passed to a convenient location for screening, settling and return to sump 32.

The thus-far enumerated structures, with the exception of drill collar 18 and the drill bit means, are conventional, and may be of any suitable construction and arrangement, and are shown to illustrate the environment of the invention and aid in a clear explanation of the operation of the preferred embodiment of apparatus hereinafter described.

While any suitable means may be employed for transmitting or conveying the information obtained at the hottom of the borehole to a location at the surface of the earth outside the borehole, the information is, in the illustrated embodiment of the system of the invention, converted into repetitive series of electric current pulses by automatically acting apparatus housed in the drill collar, the electric current pulses being translated into equivalent series of signals in the form of pressure changes created in the downwardly flowing stream of drilling fluid within the drill string, which signals are quickly transmitted therein and thereby upwardly and out of the borehole to a suitable transducer mechanism which serves to retranslate the pressure changes into sense-perceptive indications suitable for interpretation and utilization by the driller or operator. The-presspre changes in the stream of drilling fluid and which maybe wearers? produced by suitable"merE"such---as"anelectromagnetically actuated fluid valve interposed in the drilling fluid stream and controlled by the aforementioned series of electric current pulses, can be either reductions in pressure, or increases in the pressure, or combinations of the two. In either case, the pressure returns to a normal value after each pressure change. In the preferred embodiment of apparatus of the system of the invention herein disclosed, the pressure changes are chosen arbitrarily to be increases in pressure to a value somewhat above normal, followed in each case by a decrease in pressure to a normal value. Thus the pressure-changes may be considered to be pressure-variation signals. To provide for greater ease and accuracy in interpreting and evaluating the signals, the signals formed in each series may include a reference signal or pressure change of distinctive character such as of relatively long duration, and a plurality of information-representing pressure-changes or signals of shorter duration, each of which by a suitable relationship or characteristic represents information obtained by apparatus within the borehole. The characteristic or relationship of the pressure-change signals employed in the disclosed embodiment of the system of the invention for representing information is arbitrarily chosen to be the time interval between the commencement of a reference pressure-change signal and the commencement of a following information-representing pressure-change signal. In the illustrated embodiment, three different sources of information are employed in the apparatus within the borehole, and the signal-producing means is accordingly actuated by suitable regularly operating control means including a clockwork, to produce repetitive series of signals, each series including a definitely timed reference signal followed by three variably timed information-representing signals.

Referring again to Figure 1, the pressure-change signals thus propagated in the stream of drilling fluid within drill collar 18 are transmitted by and through the drilling fluid to the top of the drill string through the swivel and hose to the conduit 29 and pump 30. 'Lhg pressure changes are also transmitted through drilling fliiid contained irr'a conduit 35 connected to conduit 29 as indicated, and are transmitted past a throttling valve 36 interposed in the conduit 35 and to a transducer means indicated diagrammatically and generally at 40. The transducer may assume a variety of different structural forms, but preferably and as illustrated, comprises a fluidtight box 41 divided by a flexible diaphragm 42 into two chambers interconnected by a small bore tube 43. Conduit 35 passes through a sealed opening in the left-hand chamber and terminates in a sealed bellows 44. The right-hand chamber is provided with a bellows 45 sealed around an opening in box 41 and carrying at a sealed aperture a recorder rod 46 which is attached to diaphragm 42 for actuation thereby as indicated. Rod 46 supports and moves a pen 47 arranged to form a graph on a strip of graph paper 48 supplied from a roll 49 and taken up by a take-up roll 50 and wound on a roll 51 by suitable driving means such as a clockwork 52 in a manner and by means as indicated. Long duration or slow changes in drilling fluid pressure in conduit 35, such as are incidental to increasing depth of the borehole, are equalized at diaphragm 42 by the action of tube 43 which permits slow movements of fluid from chamber to charm her in box 41, but which is incapable of passing the fluid rapidly enough to more than slightly damp the Pressurechange signals received through conduit 35. Thus, a pressure-change signal is translated by diaphragm 42 into a corresponding movement of rod 46 and pen 47, which latter forms a sense-perceptive indication in the form of a graphical representation or record of the signal as received. As noted, the record will be, in the disclosed embodiment of the invention, a graph comprising repetitive series of indications or signals, each series comprising a relatively long reference pip P followed by three relatively short information pips P P and P The time intervals between the commencement of a reference signal or pip P and the commencement of the respective information signals or pips is translated by uniform movement of graph paper 48 by roll t? and clockwork 52 into equivalent and proportional distance intervals, the later being designated on Figure 1 as Dil, D2 and D3. As is more fully explained hereinafter in connection with Figures 4A and 4B, the distance intervals D1, D2. and D3, each representing a measure of a quantity or characteristic obtained within the borehole, are used in con junction with a time log of borehole depth in the con structure of a graphical representation of natural potential and current path resistances, or earth formation resistivities.

A time-log of borehole depth may be secured by a conventional depth vs. time meter; or the equivalent information may be supplied as indicated by a depth indicator 53 which comprises a depth measuring device provided with a printer drum rotated by a drum and cable connection to the traveling block as diagrammatically indicated, and an actuator operated periodically through a mechanical drive including a drive shaft 53d driven by clockwork 52, and arranged to print or otherwise provide depth indications at intervals on paper 48. A preferred arrangement of the apparatus is such that the depth indications are numerical indications of the actual borehole depth, and such that a printed depth indication is provided for each series of pips, as indicated in Figure 1. It will be noted that these indications slowly increase in value although due to the small increase in depth over one signal cycle successive indications are substantially the same, as shown. The transducer and signal-recording means, and the cooperating depth-indicating means, are not per se of the present invention, and may be of any suitable and conventional construction.

Referring now to Figures 213 and 5, the means and mode of obtaining information within the borehole and for producing pressure-change signals representative of the information will be explained. Drill bit means 16 comprising short sub 17a is insulated from a length of the drill collar 18 by means such as an insulation joint 55 between the sub and the lower section 18a of the drill collar and by cylindrical internal and external insulation jackets 56 and 57, respectively, secured to the inside and the outside of the drill collar 18 and sub 17a as indicated Thus the drill bit means may serve as a principal or primary insulated electrode. It will be obvious to those skilled in the art that sub 17a may be omitted from the structure, but preferably is included to facilitate changing of drill bits. Secured to the lower exterior of jacket 57 is an auxiliary or secondary electrode 5'8, preferably in the form of a series of spaced-apart and electrically insulated tightly fitting metallic sleeves; or, alternatively, a single long sleeve of similar nature. In either case the auxiliary or secondary electrode is insulated from the bit and sub and from the drill collar. In the interest of clarity, the drill bit may hereinafter be termed a primary or principal electrode, and electrode 58 may be termed a secondary or auxiliary electrode. While any suitable ground electrode or connection may be provided remote from the drill bit, the upper uninsulated reaches of the drill collar and the drill string exposed to the fluid in the torehole provide an excellent ground connection remote from the primary and secondary electrodes, and possess the advantage of presenting a very low resistance to the electrode currents. By the term remote is meant such distance away from the bit and secondary electrode that the currents flowing between the electrodes and the ground connection are required to flow through enough of the intervening earth that a principal part of the resistance of the earth current path is concentrated in or provided by the body of formation closely adjacent the electrodes. The electrodes serve to pass current supplied to them, in a manner more fully explained hereinafter, through the earth formations between and adjacent the ground connection and the primary and secondary electrodes, whereby through suitable measurements and operations information concerning one or more characteristics of the earth such as, for example, electric resistance and natural potential, may be obtained or determined. By alternately cutting off the supply of current to the secondary or auxiliary electrode and supplying such current, during respective successive short periods of time, while continuing the supply of current to the primary electrode, the current supplied to the latter electrode may, in the region closely adjacent the primary or principal electrode, be alternately allowed to flow along paths directed in substantially all directions away from that electrode, and restricted to a path or paths directed principally downwardly from the end face of the borehole, that is, in the direction of extension of the borehole; whereby information concerning both the formation drilled through and the yet undrilled formations beyond the end face of the borehole may be obtained, with the previously noted advantages to the driller. This will hereinafter be more fully explained.

Referring now to Figure 5, alternating current of suitable voltage and frequency is supplied to leads 60 and 61 by a conventional supply means indicated diagrammatically by reference character P. The power supply may be a battery and chopper arrangement, or a generator driven by a drilling fluid turbine such as is disclosed in Patent No. 2,524,031, or any other desired suitable means of conventional design.

A transformer T has its primary coil 65 connected across leads 60 and 61 so as to be continuously energized therefrom. The power supply leads 60 and 61 also serve as a primary or principal source of current for passage through the principal or primary electrode (the bit) and the earth between and adjacent the bit and the remote ground connection, the latter being designated by reference character G in Figure 5. The secondary coil 66 of transformer T serves as a secondary or auxiliary source of current for passage through the secondary or auxiliary electrode 58 and the earth adjacent and between the secondary electrode and the remote ground connection G. The principal and auxiliary currents supplied to the respective electrodes, as previously indicated, are controlled in regard to times of passage and so as to provide informaton concerning the selected characteristics of the earth formations in the vicinity of the electrodes. A preferred form of control means for performing these and other functions will next be described.

A clockwork 70 of any desired suitable type rotates a non-conductor shaft, indicated by the dotted line 71, at a slow, uniform speed, preferably of the order of onehalf revolution per minute. Secured to shaft 71 for rotation thereby in the direction indicated by the arrows, is a series of brush arms including those designated 72, 73, 74, 75, 76 and 77. The several brush arms are thus synchronously rotated during and through successive periods I, II, III and IV, corresponding to sectors of rotation as indicated in Figure 5; and during certain of the periods, are each adapted to make electrical contact with respective resistor and/ or conductor segments for controlling the time periods of several functional operations of the apparatus, as will hereinafter be made fully evident. Thus, brush arm 72 is arranged to conductively engage a conductor segment 72c during periods III and IV; and brush arm 73 similarly engages a resistor segment 73r during period II and a conductor segment 730 during periods III and IV. Brush arm 74 similarly engages a conductor segment 740 during period II; brush arm 75 engages a resistor segment 75r during period III; brush arm 76 engages a resistor segment 76r during period IV; and brush arm 77 engages a conductor segment 770 during period IV, as is evident from an examination of Figure 5. The several segmental structures are suitably mounted in conventional manner as in the form of wafers in a multi-bank rotary switch as will be evident to those skilled in the art. In the interest of clarity only one arm 77 and one segment 770 are shown, connected to electrode 58, but it will be understood that when that electrode is sectional, as indicated, each section will be provided with an individual brush arm and segment, connected in an obvious manner.

An electromagnetically operable drilling fluid valve, diagrammatically indicated as V in Figure and having an actuating magnet coil M, is interposed in the flow channel or stream path of the drilling fluid forced downwardly through the drill collar, and is operable from a normal open position to a closed or partially closed position by action of the coil M when the latter is energized. The valve is conventional in structure and returns to normal position upon deenergization of coil M. While in the preferred embodiment of apparatus disclosed and as hereinafter described, valve V moves to restrict the flow of drilling fluid upon the energization of coil M to thereby create a pressure rise in the drilling fluid stream thereabove, it will be understood that the converse type of operation may as well be employed, as hereinabove indicated; it being sufiicient for the purposes of the disclosed signalling system that a pressure change be created in the drilling fluid stream upon energization of coil M. Valve V is situated at any desired suitable location in the drill string, as, for example, in the drill collar 18 near the upper end of the annular sealed apparatus case 79 (see Figure 2a) mounted as indicated in the sectional drill collar. Valve magnet coil M has one terminal connected by a short lead 80 to one terminal of the power Supply means P and the other terminal connected by a lead 81 to the fixed contacts 82 and 831 of respective relays 82 and 83, whose corresponding movable contacts 82m and 83m are connected, as indicated, to the other terminal of power supply means P by way of lead 60. Thus, it is evident that if the normally open contacts of either relay close during any of the periods, due to deenergization of the respective relay, current will be supplied to valve coil M and valve V will be actuated to produce a pressure-change signal of the character of a pressure rise in the drilling fluid stream above the valve.

Referring again to the continuous slow rotation of shaft 71, as the brush arms move from period IV to period I, arm 72 moves off conductor segment 72c, arm 73 moves off conductor segment 73c and the sensitive relay 82, which is preferably of the nonpolarized, quick-acting, slow-release, direct-current type, becomes deenergized. As a consequence, valve magnet coil M' is energized and valve V is actuated for a relatively long period of time and produces a relatively long pressure-change signal in the drilling fluid stream, which change corresponds to and produces a reference pip P (see Figure 1) on the graph paper 48 outside the borehole. As the brush arms move from period I to period II brush arms 72, 75, 76 and 77 rotate inactive, While brush arm 73 contacts resistor segment 73r and brush arm 74 contacts conductor segment 74c. Thereby there is closed a circuit including brush arm 73, lead 85, the coil of relay 82, ground lead GG, ground connection G, the earth between ground connection G and bit 17, bit 17, lead 86, choke coil 87, potential network 88, lead 89, conductor segment 74c, brush arm 74, lead 90, conductor segment 73c, and resistor segment 73r. Potential network 88 is so constituted, as, for example, by having a battery 88b of one and a half volts, a resistor 88r of 200 ohms, and a resistor 88a of 1500 ohms, all connected as indicated, that a potential drop of about 200 millivolts will be provided across resistor 881'. Since the earth formation natural potential existing between ground connection G and bit 17 will rarely if ever exceed 200 millivolts negative, it will be evident that lead 89 will be at a positive potential. Thus, as the brush arms move to and through period II, there is applied to brush arm 73, a potential (with respect to ground) which varies from a negative value equal to the difference between the voltage across resistor 91r and the potential of lead 89, through a zero value and to a positive value equal to the potential of leads 89 and 90. Accordingly, as the arms move into period II, current will at first flow through the coil of relay 82 and open the relay contacts 82 and 82m, and the circuit through valve magnet coil M will be deenergized; relay 83 being, during periods I and II, energized, and hence open, in a manner hereinafter explained. During all periods of deenergization of valve magnet coil M, the valve will be open and thus offer no restriction to free flow of drilling fluid through the drill collar.

As the brush arm 73 moves over resistor segment 73r during period II, the potential applied across relay coil 82 increases in the algebraic sense from the mentioned negative value, to a zero value, and the time zero value is reached, relative to the commencement of period II (and hence also relative to the commencement of period I) is dependent upon the value of the natural potential of the earth between ground connection G and bit 17 choke 87 serving to exclude alternating potentials from lead 89 and the coil of relay 82. As the potential thus applied to arm 73 and across the coil of relay 82 passes through zero value in going from a negative to a positive value, the contacts of that relay will close, causing energization of coil M and the production of a pressurechange signal in the drilling fluid stream. This pressurechange signal causes production of pip P1 on graph paper 48 (Figure 1). The time interval between the commencement of period I and the commencement of the pressure-change signal is mathematically related to, and indicative of, the natural potential of the earth as measured between ground connection G and drill bit 17; and this time interval is that indicated by and proportional to distance interval D1 on the graph in Figure 1. As soon as the potential applied to the coil of relay 82 rises above zero value, that relay is again energized and allows valve V to open; and relay 82 then remains energized, first by the potential on lead 89 during the remainder of period II, and by the potential of battery 91b applied as indicated through segment 73c, arm 73 and lead on one side and through conductor segment 72c, brush arm 72 and ground lead GG on the other side, during periods III and IV. Thus relay 82 is ineffective to cause operation of the valve V during periods III and As the brush arms move from period II to period III, arm 74 moves off conductor segment 740, thus isolating choke coil 87 and lead 89; and arm 75 makes contact with resistor segment 75r, whereby the alternating current potential existing across leads 60 and 61 and across primary coil 65 of transformer T is applied through lead 94 and arm 75 on one side and leads 92, 93 and 86 on the other side, between ground lead GG and bit 17. Thus, principal electrode alternating current will flow from lead 61 to bit 17 via leads 92, 93 and 86, through the earth between and adjacent the bit and ground connection G, through ground connection G, ground lead GG, through an increasing portion of resistor segment 751', arm 75, lead 94, and through lead 95, to return power lead 641. At the same time, the potential across leads 60 and 61 is applied across the lower limbs of a bridge network 96 comprising equal resistors 96a and 9619 as lower limbs. It will be evident that the other limbs of this bridge network comprise, respectively, all or part of resistor sector 751', and the resistance of the earth between and adjacent bit 17 and ground connection G. The coil of relay 83, which is preferably of the sensitive A.-C., quick-acting, slow-release type, is connected as indicated across a diagonal of bridge network 96. As a result of the indicated connections, alternating current will normally flow from power lead 60, through lead 95, resistor 96a, the coil of relay 83, ground lead GG, the earth between ground connection G and bit 17, leads 86 and 93 and return to power lead 61 via lead 92; and relay 83 will thus normally hold its contacts open. However, as arm 75 moves along resistor segment 75r, a point or time is reached at which the resistance of that part of segment 751' between arm 75 and ground lead GG is equal to the resistance between ground connection G and bit 17, at which point the bridge network becomes balanced and current will cease to flow through the coil of relay 83. This deenergization of the coil of relay 83 allows contacts 83f and 83m to close, energizing valve magnet coil M with the resultant production of another pressure-change signal in the drilling fluid stream, and the production of pip P on the graph paper 48 as indicated in Figure l. The time interval between the commencement of period III and the time of balancing of the bridge network (the latter corresponding to the time of commencement of pip P is thus a measure of the resistance swept across by arm 75 on segment 75r, and similarly is a measure of the resistance offered by the earth formations to the principal primary current supplied to bit 17 and passing through the earth formations between and adjacent the bit and ground connection G. This measure is also mathematically related to and indicated by the time interval between the initiation of the reference pressure-change signal and the initiation of the pressure-change signal corresponding to pip P Shortly after the aforementioned deenergization of the coil of relay 83, bridge network 96 again becomes unbalanced by further movement of arm '75 along resistor segment 751', and relay 83 is again energized to open contacts 83 83m; and valve V returns to normal open condition. This condition is maintained by continued energization of the described relay coil circuit, until sometime after the end of period III.

At the end of period III brush arm 76 moves onto resistor segment 761- and then arm 75 moves off resistor 75r. Thus segment 761' replaces segment 751' in the bridge network 96. That this replacement is effected is evident,

since arms 75 and 76 are interconnected by a lead 97, as indicated. At the commencement of period IV brush arms 77 move into contact with conductor segments 770 (only one of each being shown), which are connected to the secondary or auxiliary electrode 58 by leads such as 98. Since arm 77 is connected by a lead 99 to one terminal of the secondary 66 of the transformer T and the other terminal of the secondary is connected to ground lead GG, and hence to ground connection G, the potential across the transformer secondary 66 is applied between secondary electrode 58 and the ground connection G. Hence a secondary or auxiliary alternating current will be passed through lead 99, arms 77, segments 77c, lead 98, secondary electrode 58, the earth between and adjacent electrode 58 and ground connection G, with a return to the transformer secondary 66 through ground lead GG. This auxiliary current, in flowing through the earth formations between the adjacent electrode 58 and the ground connection G, flows along a path or paths enclosed by the path or paths taken by the simultaneously flowing primary or principal alternating current which continues to pass through bit 17 during both of periods III and IV. The elfect of the imposition of the auxiliary current on the earth formations between electrode 58 and ground connection G is to force the principal current to change its path in a manner to be more fully explained in connection with Figures 3A through 3D. This change of principal or primary-current path involves a change of current-path resistance, and the resistance of the new path is measured and a comparison of the resistances, or of indications thereof, is utilized to gain extremely useful information concerning the earth formation ahead of the end face of the borehole, as will hereinafter be more fully explained.

As brush arm 76 moves onto resistor segment 76r at the commencement of period IV, the resistance of the earth formation path of the primary current passed through bit 17 changes in accordance with the current path change, and bridge network 96 remains unbalanced and relay 83 remains energized with its contacts open. As arm 76 moves over resistor segment 761', a time arrives during period IV when the bridge network is again balanced. This balance occurs when the resistance of the new path of the primary electric current flowing through bit 17 is equal to the resistance between arm 76 and the ground lead GG; and the time interval between commencement of period IV and the time the bridge network becomes thus again balanced will form or provide a measure of the resistance of the new path of the primary current through the earth. As the bridge network becomes balanced, the current passing through the coil of relay 33 again drops to zero value, and the relay contacts 83 33111 again close, causing production of another pressure-change signal in the drilling fluid stream. As before, the time interval between the commencement of period I and the initiation of this last-mentioned pressure-change signal is an indication of, and mathematically related to, the measure of the resistance of the new primary current path. This time interval is obviously proportional to distance interval D3 indicated on the graph paper 48 in Figure 1. Shortly after bridge network 96 is balanced for the second time it is again unbalanced by continued movement of arm 76, and this condition continues until some time during the next following period III, thus maintaining relay 83 energized through the latter part of period IV, all of periods I and II, and a part of period III, as hereinabove mentioned. At the end of period IV, the hereinabove described operation of the apparatus is cyclically repeated, the action continuing repetitively as long as power is supplied by the power supply means.

The fact that alternating current may be flowing at all times through the earth between bit 17 and the ground connection G has no adverse effect on the measurement of the natural potential of the earth formations between the bit and the ground connection, since only a potential balance mode of measuring resistance is involved, and since choke coil 87 excludes alternating current from the natural potential measuring circuit. The circuit con stants of the electrical apparatus may either be so chosen that the potential applied by transformer secondary 66 to the earth between electrode 58 and ground connection G is substantially the same as that applied between bit 17 and connection G, or, as indicated in Figures 3A to D, so chosen that the potential between electrode 58 and connection G is substantially greater than that applied between the bit and connection G. It will be apparent to those skilled in the art that the degree to which the current path of the current flowing from the bit electrode is directed downwardly will depend upon the repelling effect of the potential of, and the current flowing from, the auxiliary electrode; and that a sufliciently high potential on the auxiliary electrode would cause all of the current supplied to the bit to flow along a path or paths directed almost completely downwardly of the bit, that is, in the direction of extension of the borehole.

Referring now to Figures 2A and 2B, drill collar 18 comprises two sections 18a and 18b, the upper of which is threadably secured to the lower section or pipe of drill string 19 and to the lower section 18a of the drill collar. Apparatus case 79 is mounted in a recess provided in the sections 1811 and 18b, and houses the clockwork 70 and associated electrical apparatus. Insulated leads 86 and 98 carrying current to the electrodes pass through a sealed opening in case 79 and into a bore 100 formed in section 18a, which bore communicates with a channel 101 formed along the exterior of section 18a as indicated. For clarity of illustration this channel has been shown as exaggerated in depth. The leads 86 and 98 are sealed in channel 101 and are covered therein by insulation jacket 57. At suitable points such as at 102 (see Figure 2B), coaxial holes through the sections of electrode 58 and jacket 57 permit wires of lead 98 to be brought out and brazed to electrode 58. Continuation of lead 86 to the end of channel 101 and through a notch in the outer edge of insulation joint 55 permits that lead to be brazed to sub 1711 (or to bit 17 if no sub is employed). Ground connection G may be made by suitable internal connection to case 79, which is in turn grounded to the drill string by metallic contact therewith.

Referring now to Figures 3A to 3D, an illustrative example will be employed to explain the effect of the secondary current upon the primary electrode current, both during drilling of the borehole through a homogeneous formation, and as an interface between unlike formations is approched by the drill bit. In each of these figures reference number 119 represents a low-resistivity shale formation and reference numeral 120 represents a highresistivity oil sand lying beneath the shale formation. Pointed lines are employed in these figures to indicate, in a plane section including the borehole axis, approximate electrode current distribution and path or paths from the electrodes into the earth formations as the current or currents flow from the electrodes toward the ground connection G. In Figure 3A is a diagrammatically illustrated the general direction of the path or paths of the current flowing between drill bit 17 and ground electrode G during a typical period III. During that period the secondary current is not being supplied to secondary electrode 58 and the primary current flowing from drill bit 17, being free of any restriction as regards path through the earth between drill bit 17 and ground connection G, flows along paths directed generally radially outwardly in all directions from bit 17, with some possible leakage across to and through electrode 58 as indicated, and through the earth adjacent and between bit 17 and ground connection G. During a following period IV the primary electric current is supplied to bit 17 as during period III, but in addition, secondary electric current is supplied to secondary electrode 58; and, as indicated in Figure 3B, the effect of the secondary current supplied to electrode 58 is to force downwardly and change the path of the primary current to one which, in the region closely adjacent the bottom face of the borehole, is directed principally downwardly, or in the direction of an extension of the borehole, with the result that the resistance of this new path is largely furnished by the earth formation below and closely adjacent the end face of the borehole, as hereinabove indicated.

Figures 3A and 3B illustrate the current distribution or paths adjacent the lower end of the drill string while the drill bit 17 is drilling through a homogeneous formation at a considerable distance from an interface between unlike formations. Figure 30 illustrates by means of pointed lines the current paths of the primary alternating current during a typical period III as the drill bit or end face of the borehole approaches an interface between the aforementioned formations 119 and 120.

By reference to this figure it will be noted that the effect of the approach of the end face of the borehole toward a formation of higher resistivity than that being drilled through is to tend to concentrate the current flow more and more in a lateral direction from the drill bit due to the increase in resistance to current flow presented by formation 120. Figure 3D illustrates diagrammatically the paths of the currents, both primary and secondary, during a following period IV as the bottom face of the borehole approaches the interface between formations 119 and 128. Under the conditions diagrammatically depicted in Figure 3D, the primary electric current passing through drill bit 17 is again forced to take a more circuitous path by the restricting action of the secondary current flowing through secondary electrode 58. That longer path, again as in Figure 3B is, in the region closely adjacent the bottom face of the borehole, directed principally downwardly or in the direction of an extension of the borehole, and thus due to restriction in area and length, constitutes a path of considerably higher resistance than that presented to the primary electric current under the conditions depicted diagrammatically in ceptive indications or pips on the graph paper 48. As

previously indicated, distance intervals D1, D2. and D3, as indicated on Figure 1, are mathematically related to and indicative of the natural potential, and of the earth path resistances of the primary alternating current during periods III and IV, respectively. It will be understood that as the measuring and signalling cycles are repeated, each of the respective distance intervals D1, D2 and D3 will be subject to considerable variations as the natural potential and the earth path resistance encountered by the primary electric current vary. An indication of borehole depth is recorded on graph paper 48 as the borehole is extended by drilling, as is evident from examination of Figure l of the drawings. Using these borehole depth indications and the repetitive series of distance intervals or measurements D1, D2 and D3, the driller or operator constructs graphs of the nature or character of those shown in Figure 4B, plotting the indications or measures of the natural potential and of the two resistance-representing indications or measures as ordinates, against borehole depth as abscissae. The

graphs drawn in Figure 4B are those plotted using the results of the repetitive series of signals received during drilling through typical formations diagrammatically illustrated in Figure 4A. In Figure 4A formation is a low resistivity shale, formation 131 a medium resistivity gas sand, formation 132 a low resistivity shale, and formation 133 a high resistivity oil sand. It will be noted from an examination of Figure 4B that the natural potential remains relatively uniform throughout the drilling of any individual formation, varying substantially only as an interface between formations is reached and drilled through, all as indicated by the graph NP. Graph R in Figure 4B was constructed using the depth indications and the indications of the primary current path resistance obtained during periods III of the measuring cycles, while graph R was similarly constructed using the indications of the primary current path resistance obtained under the conditions relating to periods IV. It

will be noted that graph R indicates a gradual increase,

in resistance of the primary current path as the drill bit passed from formation 130 into formation 131, and that graph R also indicates a somewhat less gradual decrease in primary current path resistance as the drill bit approached the interface between formations 131 and 132, which would normally be expected as the drill bit approaches a formation of lower resistivity from one of higher resistivity. It will also be noted that by comparing graph R with graph NP, some indication in advance is afforded of the approach of the end face of the borehole toward an interface between formations of different electrical characteristics. This comparison affords very valuable information to the driller as is clearly evident. However, its value is greatly enhanced by the additional information afforded by a comparison of graph R; with graph R Due to the restriction of path of the primary alternating current during periods IV of the cyclically repeated measurements, more pronounced and earlier indications of approaching changes in formation resistance are afforded by graph R A comparison of graph R with graph R shows that the latter, with respect to borehole depth, leads graph R that is, shows changes in resistance before they show in graph R and it is by virtue of this leading effect that the driller or operator is enabled, by visual examination of the graphs, to determine and anticipate not only the nature of the change in resistivity of the formation being approached by the drill bit, but also the relative closeness of the drill bit or end face of the borehole to the yet to be reached interface between formations.

From the preceding description of a preferred embodiment of a system and method according to the invention, it is seen that the invention provides the driller or operator with means and procedures whereby there may be procured continuously and during drilling, one or more types of information concerning the characteristics of earth formations being drilled and yet to be drilled, and without materially interfering with the drilling processor equipment. While auxiliary electrode 58 may comprise but one section and the apparatus be accordingly limited to one brush arm 77 and one segment 77c, it will be evident to those skilled in the art that in some instances, such as when the borehole fluid is of low resistivity, enhanced results and less leakage of primary electrode current will be secured by using a sectional auxiliary electrode with the sections electrically insulated and each connected by an individual wire to a respective segment 77c associated with a common or multiple brush arm 77.

A full and complete disclosure of one preferred embodiment of physical apparatus and mode of operation of a system according to the invention having been herein made, it is evident that many changes in form and procedure within the scope of the invention will be apparent to those skilled in the art. It is desired therefore, not to be limited to the details of the illustrated embodiment of physical apparatus and described mode of procedure, but what is claimed is:

1. In a logging system adapted for logging earth formations adjacent to and beyond the end face of an earth borehole, the combination comprising: means including an electrode, and arranged to cause an electric current to flow from the electrode into the earth surrounding the end face of the borehole with relative freedom from spatial restriction; means acting to force said current to flow, in the region closely adjacent the end of the borehole, principally along a path or paths directed in the direction of an extension of the borehole; and means responsive to said currents to provide indications of an electrical characteristic of the paths along which said current flows.

2. In a logging system for logging earth formations adjacent to and beyond the end face of an earth borehole, apparatus comprising: a principal electrode facing the end face of the borehole; an auxiliary electrode closely adjacent said principal electrode but more distant from said end face of the borehole than said principal electrode, and electrically insulated from the principal electrode; a ground connection remote from said electrodes; means including power supply means connected between said electrodes and said ground connection, for supplying electric current to each of said electrodes simultaneously of substantially the same polarity for flow therethrough and through the earth formations adjacent and between said electrodes and said ground connection, said means including means responsive to said currents to provide an indication of the electrical characteristics of the formations beyond the end face of the borehole encountered by the current flowing through said principal electrode.

3. The combination defined by claim 2 and means for translating said indication to a location outside the borehole.

4. In a logging system for logging earth formations adjacent and beyond the end face of an earth borehole, the combination comprising: drilling means for operation in said borehole and including a principal electrode comprising a drill bit for facing and for contacting the formation at the end face of the borehole; an auxiliary electrode situated closely adjacent and above said principal electrode and electrically insulated therefrom; a ground connection remote from said electrodes; means including power supply means, connected separately between said electrodes and said ground connection, for supplying separate principal and auxiliary similarly phased electric currents to respective ones of said electrodes for flow therethrough and through the earth formations adjacent and between said electrodes and said ground connection, said means including means to measure the resistance to flow encountered by said principal electric current.

5. The combination defined by claim 4, and means for translating said measure to a location outside the borehole.

6. The combination defined by claim 5, and means for forming a graphical indication of said translated measure.

7. In an earth borehole logging system adapted for logging during drilling, and in which an electric current is caused to flow through a principal electrode comprising a drill bit and along paths radiating outwardly into the earth from the electrode in substantially all directions, and thence through the earth to a remote ground connection, the combination including said principal electrode and remote ground connection and comprising: means for supplying said current; means, including an auxiliary electrode above said principal electrode, for forcing said current to flow, in the earth closely adjacent said drill bit, principally along paths directed downwardly from said drill bit; and means for furnishing indications relative to said current under each condition of current flow.

8. In a system for logging a borehole during drilling thereof by means including a drill bit, the combination comprising: a principal electrode comprising said drill hit; an auxiliary electrode above and adjacent said drill bit; a ground connection remote from said electrodes; power means including power supply means, for periodically supplying respective different but similarly phased electric currents for flow through respective ones of said electrodes and the earth between and adjacent to said electrodes and said remote ground connection, said power means including control means for effecting supply of current to said principal electrode for two successive periods of time and to said auxiliary electrode during only one of said periods; and means to measure the resistances to fiow of the current supplied to said principal electrode during each of said periods.

9. The combination defined by claim 8, including means for translating said measures of resistances to a location outside the borehole.

10. In a logging system adapted for logging the earth formations closely adjacent the end face of a fluid-filled earth borehole during drilling of the borehole by means including a drill collar and a drill bit, the combination comprising: a principal electrode comprising said drill bit; an auxiliary electrode mounted on said drill collar above and closely adjacent said drill bit; a ground connection remote from said principal and auxiliary electrodes; means insulating said electrodes from each other and from said drill collar and said remote ground connection; power supply means including principal and auxiliary sources of similarly phased alternating current; means for connecting said principal source of alternating current between said principal electrode and said ground connection to cause a principal alternating current to IfiOW through the earth therebetween and thereadjacent; means for connecting said auxiliary source of alternating current between said auxiliary electrode and said ground connection to cause an auxiliary alternating current to flow through the earth therebetween and there-adjacent and thereby force said principal alternating current to flow, in the region closely adjacent the principal electrode, along paths directed principally downwardly from said principal electrode; and means for supplying indications of an electrical characteristic of the formations through which said principal alternating current flows under each of its conditions of how.

11. In a system for logging an earth borehole during drilling thereof, the combination comprising: a principal electrode for engaging the earth at the end face of the borehole; an auxiliary electrode for facing the borehole wall closely adjacent the end face of the borehole; a ground connection remote from said electrodes; means for supporting and for insulating said electrodes from each other and from said ground connection; power supply means including principal and auxiliary sources of similarly phased alternating current; means including control means for periodically connecting the principal source of alternating current between said principal electrode and the ground connection for two successive periods of time to cause a principal alternating current to flow through earth formations therebetween and there-adjacent, and for periodically connecting the auxiliary source of alternating current between said auxiliary electrode and the ground connection for only one of said periods of time to cause an auxiliary alternating current to fiow through earth formations therebetween and there-adjacent, and means to form signals representing the resistances to flow of said principal alternating current during each of said periods of time.

12. The combination defined by claim 11 and means for translating said signals into a graphical record outside the borehole.

13. In a system for logging an earth borehole during drilling thereof, the combination comprising: a principal electrode for engaging the earth at the end face of the borehole; an auxiliary electrode facing the borehole wall closely adjacent the end face of the borehole; a ground connection remote from said electrodes; means for sup porting and insulating said electrodes from each other and from said remote ground connection; principal and auxiliary sources of similarly phased alternating current; means including control means for periodically producing a measure of the natural potential existing between said principal electrode and said ground connection, for periodically connecting the principal source of alternating current between said principal electrode and the ground connection for two successive intervals of time to cause a principal alternating current to flow through earth formations therebetween and there-adjacent, and for periodically connecting the auxiliary source of alternating current between said auxiliary electrode and the ground connection for only one of said intervals of time to cause an auxiliary alternating current to flow through earth formations therebetween and there-adjacent; means to produce measures of the resistances to flow of said principal alternating current during each of said intervals of time; means for translating said measures into recurrent series of signals; and means for translating said recurrent series 18 of signals into a graphical record of said measures outside the borehole.

14. In a method of determining approach of the end face of an earth borehole toward an interface between earth formations of differing electrical characteristics, during drilling of the borehole, the procedure comprising: sequentially passing electric current through the earth between a location at the end face of the borehole and a location remote from the end face of the borehole; alternately during each sequence of current passage restricting the current, in the region closely adjacent the end face of the borehole, to paths of flow directed principally in the direction of extension of the borehole and through the earth beyond the end face of the borehole, and removing such restriction from the current; periodically obtaining information relative to the current thus restricted and freed from restriction and indicative of said characteristics; and comparing the information relating to the restricted current with the information relating to the current freed from restriction, as drilling proceeds, to determine approach of the end face of the borehole toward such interface.

15. In a method of determining variations in a resistance characteristic of earth formations beyond the end face of an earth borehole as drilling thereof proceeds, the procedure comprising: causing two successive electric currents to flow through the earth between and adjacent a first location at the end face of the borehole and a second location remote from the end face of the borehole, restricting one only of the two currents, in the region closely adjacent the first location, to paths of flow directed principally beyond the end face of the borehole; obtaining information relative to each of said currents, indicative of said characteristic; and comparing the information relative to one current with the information relative to the other current to determine changes with respect to said characteristic.

16. In a method of determining possible variations in an electrical characteristic of earth formations below the bottom face of an earth borehole during drilling of the borehole, the procedure comprising: during one interval of time causing a first primary electric current to flow along unrestricted paths of least resistance through the earth formations between and adjacent a location at the bottom face of the borehole and a location above and remote from the bottom face of the borehole, said paths being determined only by said current and said characteristic of the earth between said locations; during another interval of time causing a second primary electric current to flow through the earth between said locations while simultaneously restricting said second primary electric current to flow, in the region closely adjacent the bottom of the borehole, along paths directed principally downwardly of the bottom face of the borehole through the earth therebeneath; repeating the stated steps as drilling is carried on; producing repetitive senseperceptive indications of said characteristic as furnished by each of said primary currents; providing a record of borehole depth contemporaneously with production of said indications; and constructing from such indications and record, graphs showing values of the indications relative to borehole depth; whereby by comparison of said graphs, possible variations in said characteristic below the bottom face of the borehole may be determined.

17. In a method of determining possible changes in a characteristic of earth formations beyond the end face of an earth bodehole during drilling thereof, the procedure comprising: during one interval of time passing one primary electric current along paths through the earth formations between and adjacent a first location at the end face of the borehole and a ground location remote from said first location, said paths being determined only by said characteristic of the earth between and adjacent said first and ground locations; during another interval of time passing another primary electric current through the earth between said first and ground locations while simultaneously passing a secondary electric current between a location adjacent said first location and said ground location and along paths enclosed by the paths of said other primary electric current; whereby during said other interval of time said other primary electric current is constrained, in the region closely adjacent the first location, to flow principally through undrilled earth forma tions ahead of the end face of the borehole; producing respective indications of the two primary electric currents; translating said indications into sense-perceptive representations, one related to each flow of primary current; recurrently repeating the preceding procedural steps during drilling; forming graphical records of respective values proportional to said representations; and comparing said graphical records to determine any changes in said characteristic of earth formation beyond the end face of the borehole.

18. In a method of determining electrical resistance characteristics of earth formations below the bottom face of an earth borehole during drilling thereof, the procedure comprising: first causing a first primary electric current to flow relatively unconstrained along random paths through the earth formations between and adjacent a first location at the end face of the borehole, and a ground location remote from the first location, said paths being determined only by the resistance characteristics of the earth between and adjacent the first and ground locations; secondly, causing a second primary electric current to flow through the earth between said first and ground locations and simultaneously causing a secondary electric current to flow between a second location in the borehole and closely adjacent said first location, and said ground location, and along paths enclosed by the paths of said second primary electric current, whereby said second primary electric current is constrained, during the period of flow of the secondary current and in the region closely adjacent said first location, to flow principally through undrilled earth beyond the end face of the borehole; producing measures of the electrical resistances of the paths followed by each flow of said primary electric currents; translating said measures into measurerepresenting signals; translating said signals, including transmitting the signals to a location outside the borehole; forming graphical representations of the translated signals; repeating the aforementioned steps a plurality of times as drilling is carried on; constructing from said graphical representations, graphs representative of said measures with respect to borehole depth; and comparing said graphs to determine any changes in the resistances of said paths over an interval of time.

19. The method of determining in advance possible variations in resistance characteristics of earth formations beyond the end face of an earth borehole during drilling of the borehole, comprising: during drilling, repetitively and sequentially passing electric current along paths which in the region adjacent the end face of the borehole are directed principally in the direction of a continuation of the borehole, and along paths which in the region adjacent the end face of the borehole radiate in substantially all directions from the end of the borehole; measuring and forming series of indications of the respective resistances of said paths to said current; constructing graphs of the indications, relative to borehole depth, from a plurality of said series of indications; and determining by comparison of said graphs possible variations in resistance characteristics of the earth formation beyond the end face of the borehole.

20. The method of determining in advance possible variations in earth formation below the end face of an earth borehole during drilling of the borehole, comprising: sequentially measuring the resistance of a first type of electric current path which in the region closely adjacent the end face of the borehole is directed principally in the direction of extension of the borehole, and measuring the resistance of a second type of electric current path which in the region closely adjacent the end face of the borehole radiates in substantially all directions from the respective end of the borehole; repeating the sequence of measurements a plurality of times as drilling proceeds; comparing the measures of the resistance of said first type of electric current path with the measures of the resistance of said second type of electric path, and noting any change in the relationships of said measurements, indicative of a variation in earth formation being approached during drilling.

21. The method of determining approach of the end face of an earth borehole toward an interface between two different earth formations beyond the end face of the borehole during drilling and prior to drilling through such interface, which comprises: measuring an electrical characteristic of a body of the earth which in the region closely adjacent the bottom of the borehole substantially surrounds the end of the borehole; measuring the same electrical characteristic of a body of the earth which in the region closely adjacent the end of the borehole is principally beyond the end face of the borehole; repeatedly performing such measuring operations as drilling proceeds to provide respective series of such measurements; forming record indications of the respective series of measurements; and comparing the respective record indications and noting any change in the relationship of the record related to one series of measurements with respect to the record related to the other series of measurements to determine approach of the end face of the borehole toward an interface between two different earth formations.

22. Means for determining an electrical characteristic of an earth formation beyond the end face of an earth borehole, comprising: means to cause a flow of electric current between a location in the end face portion of such borehole and a remote location along paths a first group of which in the region adjacent the end of the borehole are directed principally in the direction of an extension of the borehole and a second group of which are in said region directed principally laterally of the axis of the borehole; and means for producing a signal representing a function of only that portion of said flow of electric current which flows along said first group of paths.

23. The method of determining an electrical characteristic of an earth formation beyond the end face of an earth borehole, comprising: causing a flow of electric current between a location in the end portion of such borehole and a remote location along paths a first group of which in the region adjacent the end of the borehole are directed principally in the direction of an extension of the borehole and a second group of which are in said region directed principally laterally of the axis of the borehole; and producing a signal representing a function of only that portion of said flow of electric current which flows along said first group of paths.

24. Apparatus according to claim 1 in which the last mentioned means comprises: means to provide indications related to the resistivity of the earth formations in the paths along which said current flows.

25. Means for determining an electrical characteristic of an earth formation beyond the end face of an earth borehole, comprising: means to cause a flow of electric current between a location in the end face portion of said borehole and a remote location along paths a first group of which in the region adjacent the end of the borehole are directed principally in the direction of an extension of the borehole, and a second group of which are in said region directed principally laterally of the axis of the borehole; and means for producing separate signals each representing functions of only that portion of said flow of electric current which flows along said first group and said second group of paths.

(References on following page) References Cited in the file of this patent UNITED STATES PATENTS Leonardon July 22, 1941 Lee Mar. 9, 1943 Piety May 2, 1944 Silverman Nov. 26, 1946 Mounce Feb. 4, 1947 Dillon Aug. 19, 1947 Nichols Sept. 30, 1947 22 Owen Aug. 3, 1948 D011 Apr. 8, 1952 Homer Dec. 16, 1952 Martin Aug. 25, 1953 Broding Sept. 29, 1953 D011 Feb. 16, 1954 Arps May 4, 1954 D011 Mar. 15, 1955 D011 July 5, 1955 

